There exist means for characterizing photonic devices in which two identical optical signals are coupled to the input of two different photonic circuits, and the optical output signals from the two circuits are compared.
However, the coupling of an optical signal, for example of an optical fiber from/to a photonic circuit, can be a source of significant uncertainties. Indeed, the losses of the optical signal due to the network coupler are very dependent on the alignment and on the inclination of the optical fiber with respect to the coupler.
Thus, it is difficult to obtain identical input and output couplings between the two circuits to be compared, which renders the characterization of the photonic devices difficult.